The invention relates to needling fiber structures. A particular but non-exclusive field of the invention is making plates, sleeves, or other needled preforms, e.g. of annular shape, suitable for constituting the reinforcement of composite material parts.
In well-known manner, a fiber structure for needling is advanced past a set of needles carried by a needling head, and the needles are periodically inserted into the fiber structure and then withdrawn therefrom, by imparting go-and-return motion to the needling head in a direction that extends transversely relative to the structure advance direction.
Reference can be made in particular to U.S. Pat. No. 4,790,052 which describes making needled fiber structures by successively needling layers formed by plies superposed flat or by turns wound one on another. The intended field of that patent is making fiber reinforcement for thermostructural composite material parts, and in particular carbonxe2x80x94carbon composite material parts or ceramic matrix composite material parts in which the fiber reinforcement is densified by means of a carbon or a ceramic matrix. Needled fiber structures are made of refractory fibers, typically carbon fibers or ceramic fibers, with it being possible to perform needling on the fibers while the fiber material is in a precursor state for carbon or ceramic, and after needling the precursor is transformed by heat treatment. The intended applications of the above-cited patent are brake disks or the diverging portions of rocket engines, which applications require materials that have good mechanical properties and the ability to conserve them at high temperatures.
Needling superposed layers of fibers serves to transfer fibers in the Z direction, i.e. transversely relative to the layers. This produces a structure which presents less non-uniformity and increased ability to withstand delamination, i.e. increased resistance to the layers separating due to shear forces to which they can be subjected, particularly in brake disks.
In order to perform needling over the entire surface area of a fiber structure, the structure is advanced past a needling head. When needling is performed on each new superposed layer, an advance movement is performed each time a new layer is put into place so that the needling head sweeps over the entire surface area of the most recently superposed layer.
If the fiber structure is caused to advance continuously at constant speed, then it moves transversely relative to the needles throughout the duration of needle penetration. In particular, when the structure is thick or once it has become thick, the forced advance causes the needles to bend and they can break. In addition to the fact that broken needles need to be replaced, the presence of broken needles within the fiber structure can be undesirable in subsequent use of that structure.
It might be envisaged to ensure that the structure advances very slowly, thereby minimizing the bending forces applied to the needles while they are present within the fiber structure, or else to advance the structure discontinuously so that it is stationary during needle penetration.
However those solutions present the clear drawback of considerably increasing the time and thus also the cost required for the process of fully needling the structure.
Another process for controlling advance in a needling machine is disclosed in U.S. Pat. No. 5,909,883, in which the rotational speed of calling rollers for the needled fiber structure is modulated, so that the rotational speed has different values for different positions of the needles. It is then necessary to provide a system allowing the rollers to be driven at a variable speed. Also, the variation in speed does not take into account the actual instantaneous forces applied to the needles.
The object of the invention is to propose a method of needling a fiber structure which makes it possible to resolve the problem of needles breaking without significantly penalizing the speed of the method, even with structures that are thick.
According to the invention, the instantaneous speed of advance of the fiber structure decreases in response to the resistance to advance exerted by the needles penetrating into the structure, and increases after the needles have been withdrawn, so that the force exerted on the needles by the advance of the structure is limited, but without completely interrupting advance throughout the entire duration of needles being present in the structure.
In a preferred implementation, the decrease in the speed of advance is caused directly by the resistance to advance exerted by the needles on penetrating into the structure.
When the structure is moved by means of a controlled member connected to a drive motor by a transmission, the decrease in the speed of advance can be absorbed by mechanical slack in the transmission. In which case, since the motor is driven at constant speed, the slack which is preferably resilient is automatically taken up once the needles have been withdrawn.
Thus, while the needles are penetrating, the speed of advance of the structure decreases to below a mean speed of advance corresponding to the speed of the motor, and once the needles have been withdrawn, this speed increases to above the mean speed of advance.
A measure of the torque at the level of a driving element engaged with the fiber structure may be carried out, in order to decrease the speed of the motor when the torque becomes smaller than a given threshold, as a consequence of the slowing down of the fiber structure.
In another implementation of the method, a value representative of the force exerted to drive the fiber structure is measured, the driving speed of the fiber structure is reduced when the measured value becomes equal to or larger than a first threshold value, and, after the speed has been reduced, the driving speed is increased when the measured value becomes lower than a second threshold value.
The second threshold value may be equal to or lower than the first one.
The measured value is for example representative of the torque exerted by a driving element for the fiber structure.
Another object of the invention is to provide an installation enabling the method to be implemented.
This object is achieved by means of an installation for needling a fiber structure, the installation comprising a needling head carrying a plurality of needles, a device for driving the needling head to impart reciprocating motion to the needles, a support for the fiber structure to be needled situated facing the needling head, and a device for driving the fiber structure so as to impart an advance movement thereto on said support, in which installation, according to the invention, the fiber structure drive device is designed to enable the speed of advance of the fiber structure carried by the support to decrease momentarily in response to the resistance to advance exerted by the needles penetrating into the fiber structure, without completely interrupting advance throughout the duration of the needles being present in the fiber structure.